Comparison of Trends and Low-Frequency Variability in Long-Term Surface Temperature Data Sets


A recent paper compares linear trends in surface air temperatures from three global data sets for the period 1958-2001. Two of these, ERA-40 and NCEP NCAR re-analyses products are derived using European and US climate models in conjunction with observational data respectively. The third data set, CRU, is derived directly from monthly station data observations. The results indicate the three data sets are in reasonable agreement, particularly for the period 1979 onwards, as the number of observations being ingested in ERA-40 and NCEP NCAR re-analyses increased substantially. Linear trends computed over the entire period are generally lower in the European re-analyses product, ERA-40 but there is agreement to within ~ 10% in the rate of warming of the terrestrial Northern Hemisphere since the late 1970s. The utility in documenting and understanding climatic trends and low-frequency variations based on ERA-40 and NCEP NCAR re-analyses has a matter of debate since atmospheric models used in producing these data sets are prone to biases. Observational coverage is limited and not without bias as well, further confounding the issue of identifying climatic trends based on such data sets. This study identifies factors influencing each of the reanalysis products, thereby aiding the judicious interpretation and application of its results.


The work was sponsored by DOE BER (PI Phil Jones/Univ East Anglia UK), European Union and Fujitsu Ltd.


Simmons et al. Comparison of Trends and low-frequency variability in CRU, ERA-40, and NCEP/NCAR analyses of surface air temperature J. Geophys. Res, 109, D24115, doi:10.1029/2004JD005306, 2004.